Revert "Make compiler/optimizing/ symbols hidden."

This reverts commit e2727154f25e0db9a5bb92af494d8e47b181dfcf.

Reason for revert: Breaks ASAN tests (ODR violation).
Bug: 142365358

Change-Id: I38103d74a1297256c81d90872b6902ff1e9ef7a4
diff --git a/compiler/optimizing/code_generator_utils.cc b/compiler/optimizing/code_generator_utils.cc
index 6dec968..dd47a1f 100644
--- a/compiler/optimizing/code_generator_utils.cc
+++ b/compiler/optimizing/code_generator_utils.cc
@@ -20,7 +20,7 @@
 
 #include "nodes.h"
 
-namespace art HIDDEN {
+namespace art {
 
 void CalculateMagicAndShiftForDivRem(int64_t divisor, bool is_long,
                                      int64_t* magic, int* shift) {